Helper class to provide a typed, closure based KVO interface in Swift

TypedKVOClosure.swift

import Foundation

struct Change<T>: CustomDebugStringConvertible {
    let oldValue, newValue: T?
    
    var debugDescription: String {
        let prettyString: T? -> String = { return $0 != nil ? "\($0!)" : ".None" }
        return "ChangeType:\n\tOld value: \(prettyString(oldValue))\n\tNew value: \(prettyString(newValue))"
    }
}

final class Observer<T>: NSObject {
    
    private let context = UnsafeMutablePointer<Void>()
    typealias ChangeBlock = Change<T> -> Void
    weak var object: AnyObject?
    var keyPath: String
    private var changeBlock: ChangeBlock
    private var tornDown = false
    
    init(object: NSObject, keyPath: String, changeType: T.Type, onChange: ChangeBlock) {
        self.object = object
        self.keyPath = keyPath
        changeBlock = onChange
        super.init()
        object.addObserver(self, forKeyPath: keyPath, options:[.New, .Old], context: context)
    }
    
    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        guard keyPath == self.keyPath && context == self.context else { return }
        changeBlock(Change<T>(oldValue: change?["old"] as? T, newValue: change?["new"] as? T))
    }
    
    private func tearDown() {
        object?.removeObserver(self, forKeyPath: keyPath, context: context)
        tornDown = true
    }
    
    deinit {
        assert(tornDown, "Deinit called wihtout calling tearDown first")
    }
    
}

// Usage:

class Person: NSObject {
    dynamic var name: String?
}

let max = Person()

let observer = Observer(object: max, keyPath: "name", changeType: String.self) { change in
    print(change)
}

max.name = "Max"
max.name = "Bruno"
max.name = "Pierre"

observer.tearDown()